1. Field of the Invention
The present invention relates to a demultiplexer, in particular for a so-called set top box (STB) for decoding digital signals transmitted over a cable and/or satellite network.
2. Description of the Related Art
Standards for broadcasting digital video have been established in recent years. These standards have utilised MPEG-2 transport streams with audio and video compression technologies and data packet formatting (see ISO-IEC 13818-1,2 and 3). The initial implementations of digital transmission over networks were to be used in xe2x80x9cbroadcastxe2x80x9d transmissions where the same signal was to be transmitted to all receivers in the network. However, it is expected that xe2x80x9cnarrowcastxe2x80x9d transmissions will be introduced such that a signal is transmitted over the network for reception by one receiver only. This technique is particularly applicable to so-called xe2x80x9cvideo-on-demandxe2x80x9d. It is therefore necessary to provide set top boxes which are capable of receiving both broadcast and narrowcast (e.g. Asynchronous Transfer Mode, ATM) signals.
Broadcast (e.g. MPEG-TS) and narrowcast signals are encoded in different formats and require separate decoders to convert the transmitted signals back to the basic MPEG video signal. This is not desirable as it adds to the cost of the set top box and the complexity of manufacture. As the set top box is intended to be for mass production, it is clearly desirable to minimise the cost while still providing the facility to receive narrowcast-type services.
According to the present invention there is provided a demultiplexer for selectively demultiplexing ATM type signals and MPEG type signals received at an input of the demultiplexer, the demultiplexer comprising:
an MPEG demultiplexer for demultiplexing an MPEG transport stream;
an ATM section for identifying MPEG transport stream data in an ATM type signal; and
means for selectively connecting the input to the MPEG demultiplexer or the ATM section according to whether the received signal is an MPEG type signal or an ATM type signal; wherein
the demultiplexer comprises a memory shared between the demultiplexer and the ATM section and the ATM section processes ATM type signals by storing the received signals in the memory and providing the MPEG demultiplexer with addresses appropriate for the MPEG demultiplexer to retrieve demultiplexed data from the memory.
According to the present invention, there is also provided a method of demultiplexing ATM type signals comprising;
storing the signal data in a memory;
examining the stored signal data;
identifying MPEG transport stream data in the stored signal data by means of addresses; and
recovering demultiplexed MPEG data by retrieving appropriate stored data on the basis of said addresses.
In this way, processing efficiency may be dramatically improved. In contrast to previous proposals, the data need not be successively read, processed and rewritten, since only pointers to the common memory need be passed between consecutive processors. Furthermore, when demultiplexing ATM type signals, both processing sections need not operate at full power, since some of the processing conducted by the ATM section can be made use of by the MPEG demultiplexer thereby reducing its operations. Finally, the overall memory requirements are reduced.
Preferably, the demultiplexer is provided as a single integrated circuit. This further reduces the need for input/output pins in the overall set-top box circuit.